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Abstract

In order to clarify the controversies of hardening mechanism for TiN/SiNx-based nanocomposite films, the microstructure and hardness for TiN/SiNx and TiAlN/SiNx nanocomposite films with different Si content were studied. With the increase of Si
content, the crystallization degree for two series of films firstly increases and
then decreases. The microstructural observations suggest that when SiNx interfacial phase reaches to a proper thickness, it can be crystallized between adjacent
TiN or TiAlN nanocrystallites, which can coordinate misorientations between nanocrystallites
and grow coherently with them, resulting in blocking of the dislocation motions and
hardening of the film. The microstructure of TiN/SiNx-based nanocomposite film can be characterized as the nanocomposite structure with
TiN-based nanocrystallites surrounded by crystallized SiNx interfacial phase, which can be denoted by nc-TiN/c-SiNx model ('c’ before SiNx means crystallized) and well explain the coexistence between nanocomposite structure
and columnar growth structure within the TiN/SiNx-based film.

Keywords:

TiN/SiNx film; Nanocomposite; Hardening mechanism; Microstructure

Background

As superhard (hardness H ≥ 40 GPa) film material, nanocomposite films have been widely investigated in the
past decades for use as wear-resistant coatings on tools and mechanical components
[1,2]. Among them, the pseudobinary TiN/SiNx is a representative film due to strong surface segregation of the constituent phases
(TiN and SiNx have essentially no solid solubility). Especially, since hardness as high as 80 to
105 GPa was reported by Veprek et al. in 2000 [3], it has attracted much attention from the scientific community. So far the nanostructure
and hardening mechanism have been widely explained by nc-TiN/a-SiNx model proposed by Veprek et al. in 1995 [4], in which equiaxed TiN nanocrystallites (nc-TiN) were embedded in an amorphous SiNx (a-SiNx) matrix.

However, this model is in dispute due to the lack of direct experimental evidence,
which mainly reflects in two aspects. On one hand, whether TiN crystals are transformed
from columnar crystals into equiaxed nanocrystallites is disputed, since there was
no direct cross-sectional transmission electron microscopy (TEM) observation for the
isotropic nature of the TiN grain. On the other hand, whether SiNx phase exists as amorphous state is also disputed, since Veprek et al. [4] suggested SiNx was amorphous because no obvious SiNx Bragg reflections in X-ray diffraction (XRD) patterns were found, which lacked direct
observational evidence so far. Later, based on their high-resolution TEM (HRTEM) observations,
Kong et al. [5] reported that TiN were columnar nanocrystals, rather than equiaxed nanocrystals,
separated by crystallized SiNx interfacial phases. Hultman et al. [6] suggested that SiNx interfacial phase could be crystalline located around TiN nanocrystals according to
their ab initio calculations. However, they did not give direct experimental evidence. In addition,
the cross-sectional TEM published by Zhang et al. [7] and Kauffmann et al. [8] showed that even with increased Si content up to 12 at.%, the TiN/SiNx nanocomposite films still had a columnar morphology, which increases the uncertainty
of the existing model and hardening mechanism of TiN/SiNx film.

To clarify these controversies about hardening mechanism, TiN/SiNx and TiAlN/SiNx nanocomposite films with different Si content were synthesized since the hardness
of TiN/SiNx-based nanocomposite films was highly sensitive to the thickness of SiNx interfacial phase [3,4]. The relationship between microstructure and hardness for two series of films would
be studied. Special attention would be paid to the morphology and structure of constituent
phases in two films.

Methods

Materials

The TiN/SiNx and TiAlN/SiNx nanocomposite films were fabricated on the silicon substrates by reactive magnetron
sputtering system. The TiN/SiNx and TiAlN/SiNx nanocomposite films were sputtered from TiSi and TiAlSi compound targets (99.99%),
respectively, with 75 mm in diameter by RF mode and the power was set at 350 W. The
TiSi and TiAlSi compound targets with different Si content were prepared by cutting
the Ti (at.%, 99.99%), TiAl (Ti at.%/Al at.% = 70%:30%) and Si targets (at.%, 99.99%),
respectively, into 25 pieces and then replacing different pieces of Ti and TiAl with
same piece of Si. Adopting this method, TiSi and TiAlSi targets with different Si/Ti
(or Si/Ti0.7Al0.3) volume or area ratios, including 1:24, 2:23, 3:22, 4:21, and 5:20 were prepared.
The base pressure was pumped down to 5.0 × 10-4 Pa before deposition. The Ar and N2 flow rates were 38 and 5 sccm, respectively. The working pressure was 0.4 Pa and
substrate was heated up to at 300°C during deposition. To improve the homogeneity
of films, the substrate was rotated at a speed of 10 rpm. The thickness of all the
TiN/SiNx and TiAlN/SiNx nanocomposite films was about 2 μm.

Characterization

The microstructures of TiN/SiNx and TiAlN/SiNx nanocomposite films were characterized by XRD using a Rigaku D/MAX 2550 VB/PC (Rigaku
Corporation, Tokyo, Japan) with Cu Kα radiation and field emission HRTEM using a Philips
CM200-FEG (Philips, Amsterdam, Netherlands). The preparation procedures of cross-section
specimen for HRTEM observation are as follows: The films with substrate were cut into
two pieces and adhered face to face, which subsequently cut at the joint position
to make a slice. The slices were thinned by mechanical polishing followed by argon
ion milling. The hardness was measured by a MTS G200 nanoindenter (Agilent Technologies,
Santa Clara, CA, USA) using the Oliver and Pharr method [9]. The measurements were performed using a Berkovich diamond tip at a load of 5 mN
with the strain rate at 0.05/s. The indentation depth was less than one-tenth of the
film thickness to minimize the effect of substrate on the measurements. Each hardness
value was an average of at least 16 measurements.

Results and discussion

The XRD patterns of TiN/SiNx and TiAlN/SiNx nanocomposite films with different Si content are shown in Figure 1. It can be seen that two series of films are only composed of TiN or TiAlN phase,
while no SiNx phase is detected. Veprek had attributed the absence of SiNx phase to its amorphous characteristic [4]. Actually, it can also be explained by low content of SiNx phase. Figure 1a,b indicates that TiN/SiNx and TiAlN/SiNx nanocomposite films both present (200) preferred orientation. With the increase of
Si content, the intensities of TiN and TiAlN (200) diffraction peaks firstly increase
and then decrease, suggesting that the crystallinity for TiN and TiAlN phases initially
improves and then deteriorates. The TiN/SiNx and TiAlN/SiNx films exhibit the highest crystallinity when Si/Ti (or Si/Ti0.7Al0.3) ratio is 4:21 and 3:22, respectively.

The influence of Si content on crystallinity throws doubt upon the nc-TiN/a-SiNx model proposed by Veprek [3,4]. If SiNx phase exists as amorphous state, the increase of Si/Ti ratio from 1:24 to 5:20 (SiNx fraction accordingly rises from 4 to 20 at.%) only leads to thickening of amorphous
SiNx interface, which cannot improve the crystallization degree of film, but lowers it
due to the increasing impeditive effect on TiN growth. In addition, as amorphous SiNx interfacial phase thickens, TiN and TiAlN phases cannot only present (200) orientation,
but may also grow along other directions owing to the randomicity of crystallite growth
[10]. Therefore, whether SiNx interfacial phase is amorphous deserves to be further deliberated.

In fact, the effect of Si content on crystallinity of TiN/SiNx and TiAlN/SiNx films brings into our mind the influence of amorphous modulation layer thickness
on the crystallization degree of nanomultilayered films, such as TiN/SiC [11], TiAlN/SiO2[12], and CrAlN/SiNx[13]. In these nanomultilayered film systems, with the increase of amorphous layer thickness,
the crystallization degree of films firstly increases and then decreases, which can
be attributed to two facts. On one hand, the initial increase of amorphous layer thickness
could not only crystallize the amorphous layer and grew epitaxially with crystal layer,
but also the newly deposited crystal layer could grow epitaxially on crystallized
amorphous layer, leading to the 'mutual promotion effect’ of growth in nanomultilayers
and improvement of crystallization integrity. The thicker the crystallized amorphous
layer thickness is, the higher the crystallization degree of the nanomultilayered
film. On the other hand, with further increase of amorphous layer thickness, the amorphous
layers cannot keep the crystallization state and change back into the amorphous state,
which destructs epitaxial growth structure and decreases the crystallization integrity
of the nanomultilayer. Therefore, it can be deduced that with increase of Si content,
SiNx interfacial phase may undergo the crystallized process, helping to improve the crystallization
degree and maintain (200) preferred orientation of TiN/SiNx and TiAlN/SiNx films. Nevertheless, this deduction needs to be verified from HRTEM observations.

Figure 2 presents the typical cross-sectional HRTEM images of TiN/SiNx film with Si/Ti ratio of 4:21 and TiAlN/SiNx film with Si/Ti0.7Al0.3 ratio of 3:22. From Figure 2a,c, it is clear that similar with TiN and TiAlN monolithic films, both nanocomposite
films show obvious columnar growth structure, in agreement with results from Zhang
et al. [7] and Kauffmann et al. [8]. This columnar growth structure cannot be explained by the nc-TiN/a-SiNx model. According to the model [4,14], with the addition of Si element, the amorphous SiNx interfacial phase interrupted the columnar growth of TiN and divided into many equiaxed
TiN nanocrystallites. In this case, the columnar cross-sectional morphology should
not be observed, but the amorphous fracture morphology as presented in [15]. However, the columnar cross-sectional structure is obviously observed in both TiN/SiNx and TiAlN/SiNx films, which brings further question to nc-TiN/a-SiNx model.

Figure 2.Cross-sectional HRTEM images of TiN/SiNx and TiAlN/SiNx nanocomposite films. (a) Low magnification, (b) medium magnification, (d) high magnification for TiN/SiNx nanocomposite film (Si/Ti = 4:21), and (c) low magnification, (e) high magnification for TiAlN/SiNx nanocomposite film (Si/Ti0.7Al0.3 = 3:22). The SiNx interfacial phase is observed to exist as crystallized state, rather than amorphous
state, such as E zone between A and C crystals, F zone between A and B crystals, H
zone between B and D crystals, and G zone between C and D crystals.

From Figure 2a, it can also be seen that many small equiaxed nanocrystallites exist within the
TiN/SiNx film in the cross-sectional morphology. In the medium-magnification image of Figure 2b, it is obvious that many equiaxed nanocrystallites with dark contrast are surrounded
by the interfacial phase with bright contrast. According to the nanocomposite structure
of TiN/SiNx film, it is not difficult to conclude that the equiaxed nanocrystallites with dark
contrast and interfacial phase with bright contrast correspond to TiN and SiNx phases, respectively, indicating that the film constituted of equiaxed TiN nanocrystallites
encapsulated with SiNx interfacial phase, rather than columnar-like TiN nanocrystals proposed by Kong et
al. [5]. The average size of TiN crystallite is about 6 to 10 nm, with average SiNx interfacial thickness of 0.5 to 0.7 nm. In the high-magnification image of Figure 2d, the TiN crystallites basically grow along with the same direction and present the
epitaxial growth structure. The SiNx interfacial phase is observed to exist as crystallized state, rather than amorphous
state, such as the E zone between A and C crystals, F zone between A and B crystals,
H zone between B and D crystals, and G zone between C and D crystals, which verifies
the validity of above deduction. It is worth noting that there are also some amorphous
areas present in Figure 2d. Actually, these regions are not composed of real amorphous phase. When we slightly
tilted the specimen, the regions that appeared to be amorphous could change into crystallized
structure, which suggested that there existed the misorientation difference between
different regions and that the 'amorphous’ regions are not really composed of amorphous
phase, but crystallized phase. Therefore, it is reasonably believed that there exists
the same 'crystallized effect’ of nanomultilayered films in nanocomposite films, namely,
when Si content increases to an appropriate value, that is, SiNx interfacial phase reaches to a proper thickness, the SiNx interfacial phase can be crystallized under the template effect of adjacent TiN crystallites,
which can coordinate the misorientations between TiN crystallites and grow coherently
with them. In high magnification of TiAlN/SiNx film, it can also be observed that the lattice fringes continuously go across adjacent
TiAlN crystallites through SiNx interfaces, suggesting that SiNx phase has been crystallized between adjacent TiAlN crystallites and grows coherently
with them (Figure 2e). Comparatively, the SiNx interfacial thickness of TiAlN/SiNx film is smaller (about 0.3 to 0.5 nm) based on Figure 2e than that (about 0.5 to 0.7 nm) of TiN/SiNx film in Figure 2d, which is agreement with the fact that the Si/Ti0.7Al0.3 ratio of 3:22 in TiAlN/SiNx film is lower than Si/Ti ratio of 4:21 in TiN/SiNx film.

Figure 3 shows that the typical cross-sectional HRTEM images of TiN/SiNx nanocomposite film with Si/Ti ratio of 5:20. It can be seen from Figure 3a that the thickness of SiNx interfacial phase increases compared with Figure 2b (Si/Ti = 4:21). The average size of TiN crystallite is about 4 to 8 nm, smaller
than that in Figure 2b (6 to 10 nm). From the high-magnification image in Figure 3b, it can be seen that SiNx interfacial phase presents amorphous state, rather than crystallized state, suggesting
that SiNx interfacial phase cannot maintain the crystallization with high interfacial phase
thickness and transforms back into amorphous state. Amorphous SiNx interfacial phase breaks the epitaxial growth structure between the adjacent TiN
nanocrystallites, leading to the various growth misorientations for TiN nanocrystallites,
as shown in Figure 3b.

According to the above analysis, TiN/SiNx and TiAlN/SiNx nanocomposite films have the same interfacial morphological evolution with nanomultilayered
films. If this is a fact, the TiN/SiNx and TiAlN/SiNx nanocomposite films should be effectively strengthened when SiNx interfacial phase is well crystallized and the film presents the highest crystallization
degree. Figure 4 shows the dependence of hardness for two films on Si content. It can be seen that
the hardness values for two films both firstly increase and then decrease with increase
of Si content. TiN/SiNx and TiAlN/SiNx films achieve the maximal hardness values of 43.7 and 38.4 GPa, respectively, with
Si/Ti (or Si/Ti0.7Al0.3) ratio of 4:21 and 3:22, which validates our deduction.

Figure 4.Variation of hardness of TiN/SiNx and TiAlN/SiNx nanocomposite films with change of Si content.

It is not difficult to find that the variation of hardness with increase of Si content
is in accord with crystallization degree. According to the hardening mechanism proposed
in nc-TiN/a-SiNx model [3,4,14], the TiN crystallite size is too small for dislocation activities, and the film can
only deform by grain boundary sliding (i.e., by moving single undeformed TiN nanocrystallites
against each other). However, based on this mechanism, TiN nanocrystallites that slide
along grain boundary must cause the coordinate movement of adjacent nanocrystallites,
such as crystallite rotation and shift [16], and leave trace in the sliding boundary, which both lack direct experimental evidence
from the existing literatures. In addition, the dependence of hardness on Si content
should not have related to crystallization degree.

Actually, we believe that with the initial increase of Si content, SiNx interfacial phase with low thickness inclines to grow epitaxially on the surface
of TiN nanocrystallites in order to lower the interfacial energy between TiN and SiNx [17]. When the newly arriving TiN deposits on SiNx surface, it inclines to grow along the original direction. As a result, SiNx interfacial phases present to be crystallized, transferring the growth direction
and maintaining the epitaxial growth structure between the adjacent TiN nanocrystallites,
as shown in the schematic diagram of Figure 5a. In this case, the nanocomposite film can exhibit the characteristic of nanomultilayered
films in the local area, as shown in Figure 5a. According to Koehler's modulus difference strengthening theory [18], when the dislocations traverse across the coherent interface in nanomultilayer,
the dislocation motions are hindered at interface by the force that is generated from
the two layers with different shear moduli, which can effectively strengthen the film.
Furthermore, the compressive and tensile stress fields are created at the coherent
interface due to the difference of lattice parameter between two layers, which can
also block the movement of dislocations and be partially responsible for the hardening
effect [19]. It is worth noting that due to the low crystallization degree at low Si content,
the epitaxial growth structure is not well formed. Therefore, the impeding effect
of coherent interface on dislocation motion decreases, resulting in the comparatively
low hardness of film with low Si (Si/Ti ratio is below 4:21 or Si/Ti0.7Al0.3 ratio is below 3:22). The epitaxial growth between TiN or TiAlN and SiNx cannot disturb the columnar growth structure due to the improvement effect on crystallization
degree, but can exist inside the columnar crystals of TiN/SiNx and TiAlN/SiNx films, as illustrated in Figure 5b; namely, the nanocomposite structure with TiN nanocrystallites surrounded by crystallized
SiNx interfacial phase (expressed as nc-TiN/c-SiNx model, 'c’ before SiNx means crystallized) and columnar growth structure can coexist within the TiN/SiNx film, which is a typical difference from former nc-TiN/a-SiNx model.

Figure 5.Cross-sectional schematic diagrams. (a) Nanoscale configuration (nc-TiN/c-SiNx model) and (b) columnar crystals within TiN/SiNx nanocomposite film (the red frame and the dash line show that (a) is the microstructural model of the local zone within (b)).

Nevertheless, with further increase of Si content, the SiNx interfacial phase thickens and cannot maintain the crystallized state between adjacent
TiN nanocrystallites, resulting in the transformation back into the amorphous state
and breakage of epitaxial growth structure. Accordingly, the blocking effects on the
dislocation motions decrease. Despite that the amorphous phase can also act as an
obstacle for dislocation movement, its impeding effect on the dislocation motion is
much smaller than that of coherent interface. Therefore, the hardness of the film
decreases. It is worth noting that the Si/Ti ratio at which film presents the highest
crystallinity and hardness for TiAlN/SiNx film is 3:22, lower than that of 4:22 for TiN/SiNx film. That is to say, the maximal crystallized SiNx interfacial thickness maintained by TiAlN is smaller than that by TiN, which can
be attributed to the misfit difference between TiN/SiNx and TiAlN/SiNx [14]. The lattice parameter of TiN decreases with the addition of Al [20], resulting in the increase of misfit between TiAlN and SiNx, which reduces the epitaxial breakdown thickness of SiNx and might also be the reason for lower maximal hardness for TiAlN/SiNx film relative to TiN/SiNx film.

Conclusions

In summary, in order to clarify the controversies of hardening mechanism for TiN/SiNx-based nanocomposite films, the microstructure and hardness for TiN/SiNx and TiAlN/SiNx nanocomposite films with different Si content were studied. With the increase of
Si content, the crystallization degree for two series of films firstly increases and
then decreases. The microstructural observations suggest that when SiNx interfacial phase reaches to a proper thickness, it can be crystallized between adjacent
TiN or TiAlN nanocrystallites, which can coordinate misorientations between nanocrystallites
and grow coherently with them, resulting in blocking of the dislocation motions and
hardening of the film. The microstructure of TiN/SiNx-based nanocomposite film can be characterized as the nanocomposite structure with
TiN-based nanocrystallites surrounded by crystallized SiNx interfacial phase, which can be denoted by nc-TiN/c-SiNx model ('c’ before SiNx means crystallized) that can well explain the coexistence between the nanocomposite
structure and columnar growth structure within the TiN/SiNx-based film.

Competing interests

The authors declare that they have no competing interests.

Authors' contributions

WL designed the experiment and wrote the article. PL, YZ, and FM carried out the synthesis
of TiN/SiNx and TiAlN/SiNx nanocomposite films. XL, XC, and DH assisted in the technical support for measurements
(XRD, HRTEM, and nanoindention) as well as the data analysis. All authors read and
approved the final manuscript.